Triarylmethane compounds are well known to those skilled in the art, finding use in a variety of applications, such as for example, as textile dyestuffs and as fungicides. U.S. Pat. No. 501,104, for example, discloses 4,4'-bis(dimethylamino)-4"-carboxytriphenylmethane; U.S. Pat. No. 4,041,054 discloses 4-halogeno-4',4"-diarylaminotriphenylmethanes; U.S. Pat. No. 3,647,349 discloses carbonium ion salts based on 4-substituted amino-4'-alkoxytriphenylmethanes containing at the 4" position a group such as an alkyl, aralkyl, aryl, alkoxy, aralkoxy, aryloxy, alkylmercapto, arylmercapto, halogen, carboxylic acid ester radical, carbonamido, sulfonamido, cyano, nitro, alkylsulfonyl, or other groups; and U.S. Pat. No. 3,794,642 discloses triphenylmethanes wherein each of the phenyl rings may be substituted with a halogen, C.sub.1 -C.sub.4 alkyl, nitro, amino, cyano, acetyl, methoxy, or trifluoromethyl group.
Recently, various triarylmethanes have been suggested for use as organic photoconductors in photoconductive elements. For example, U.S. Pat. No. 4,047,949 discloses that 4,4'-bis(diethylamino)-2,2'-dimethyltriphenylmethane is suitable for use as a photoconductor for electrophotoconductive elements. Other triarylmethanes which have been suggested for use as photoconductors include 4',4"-diamino-4-dimethylamino-2',2"-dimethyltriphenylmethane, 4',4"-bis(diethylamino)-2,6-dichloro-2',2"-dimethyltriphenylmethane, 2',2"-dimethyl-4,4',4"-tris(dimethylamino)triphenylmethane, 4',4"-bis(diethylamino)-4-dimethylamino-2',2"-dimethyltriphenylmethane, 4',4"-bis(diethylamino)-2-chloro-2',2"-dimethyl-4-dimethylaminotriphenylme thane, 4',4"-bis(diethylamino)-4-dimethylamino-2,2',2"-trimethyltriphenylmethane, 4',4"-bis(dimethylamino)-2-chloro-2',2"-dimethyltriphenylmethane, 4',4"-bis(dimethylamino)-2',2"-dimethyl-4-methoxytriphenylmethane, 4',4"-bis(benzylethylamino)-2',2"-dimethyltriphenylmethane and 4',4"-bis(diethylamino)-2,2"-diethoxytriphenylmethane (U.S. Pat. Nos. 3,703,371 and 3,703,372); the hydroxy, halo, nitro, cyano, sulfo, alkoxy, carboxy, alkyl, acyl, acylamido or acyloxy substituted 4,4'-bis(dialkylamino)triphenylmethanes of U.S. Pat. No. 3,739,000; and the 4',4"-bis(disubstituted amino)-2',2"-disubstituted triphenylmethanes of U.S. Pat. No. 3,542,547. This latter group of compounds may be optionally substituted at the 4 position with a dialkylamino, alkylamino, amino, alkyl, alkoxy, hydroxyl or halogen group, or at the 5' and 5" positions with an alkyl, alkoxy, hydroxyl or halogen group.
Other classes of compounds in addition to the triarylmethanes which have heretofore been suggested for use as photoconductors in electrophotographic elements are described in U.S. Pat. Nos. 3,703,371; 3,703,372; and 4,140,529.
In order for a given compound to be suitable for use as an organic photoconductor in photoconductive elements, photoconductive elements containing the compound should exibit desirable photographic speeds, and for many applications be stable to ambient light, such as the 50-100 foot-candle lighting normally encountered, for example, in a business office, non-safe processing facility or the like. In addition, the photoconductor compound should exhibit a low tendency to bloom, i.e., a low tendency to migrate to the surface of the photoconductive element. Both pre-exposure fatigue (the limit of tolerance of a photoconductor compound for ambient light) and blooming have adverse effects on the photographic sensitivity, reducing a photoconductive element's imaging ability and speed.
As is well known to those skilled in the art, photoconductive elements, such as electrophotographic film, typically comprise a support having coated thereon a photoconductive composition comprising an insulating binder or matrix resin, an organic photoconductor and a sensitizing dye. The pre-exposure light fatigue of an electrophotographic film based on organic photoconductors has been found to depend on all three main components of the photoconductive layer, but the photoconductor appears to exert the dominant influence on the stability of the sensitizing dye. It has also been found that the loss of photospeed of a photoconductive element containing a given sensitizer upon pre-exposure illumination is closely related to the type of photoconductor employed, such as for example, phenylenediamine, oxydianiline or triarylmethane. It is thus desirable to employ as the organic photoconductor compounds which have a high tolerance for ambient light. This tolerance for ambient light is especially valuable in demanding electrophotographic applications requiring controlled sensitometry wherein repeated variable exposure to ambient lighting occurs, such as is involved in the use of the film as a microfilm file record with an add-on updating capability. Accordingly, as used herein, the term "pre-exposure fatigue resistance" refers to the tolerance of a photoconductor compound for ambient light, such as the 50-100 foot-candle lighting normally encountered in most indoor environments. Under the foregoing conditions, conventional electrophotographic films based on organic photoconductors such as, for example, the phenylenediamines can be expected to suffer enough sensitivity loss due to pre-exposure fatigue in just two hours to noticeably affect image quality.
In addition to pre-exposure fatigue resistance, photoconductors suitable for use in electrophotographic elements must exhibit a low tendency to bloom or migrate to the surface of the photoconductive layer. Compounds having a pronounced tendency to bloom will, particularly upon storage, rise to the surface of the polymer matrix of the photoconductive layer and form liquid or crystalline deposits. Such blooming has a deleterious effect on image quality.
In contrast to the compounds of the present invention, the triarylmethanes which have heretofore been suggested for use as photoconductors for electrophotographic elements do not possess that combination of pre-exposure fatigue resistance and blooming properties desirable for commercial use as photoconductors in many electrophotographic systems. While the triarylmethanes of the prior art have performed satisfactorily in certain electrophotographic elements wherein these compounds are utilized in conjunction with certain specific sensitizers and binder resins, with other common sensitizer and binder resin combinations the triarylmethanes of the prior art have exhibited less than desirable pre-exposure fatigue resistance and blooming properties. As the triarylmethanes exhibit a desirable photoconductive efficiency, it would thus be desirable to provide a new class of triarylmethane compounds which possess the pre-exposure fatigue resistance and blooming properties desirable for successful use as photoconductors in a wide variety of electrophotographic elements.